1. Field of the Invention
This invention relates to a bellows-form boot used for covering a universal joint.
2. Related Art Statement
Wheel shafts of an automobile are driven to rotate by means of driving shafts through universal joints each housed in a joint case. In order that extraneous matter such as sand, dust, etc. may not enter into the joint case, a bellows-form boot moulded of an elastomeric material like rubber is attached to cover between the joint case and the driving shaft on the connection side and further, a grease is sealed within the boot thereby to bring the universal joint into lubrication.
Heretofore, for example, a universal joint boot as illustrated in FIG. 3 is known which is shaped integrally of a large-diameter cylindrical portion (3) to be fitted on the outer periphery of a joint case (2) on the wheel (1) side, a small-diameter cylindrical portion (5) to be fitted on the outer periphery of a driving shaft (4) and a bellows portion (7) interconnecting between the former and the later, which portion has a constant wall thickness over the whole length of it and includes 3 to 6 ridge portions (6), the large-diameter portion (3) and the small-diameter portion (5) being fastened fixedly with fastening members (8, 8') such as retaining band. One example of such type of boot for universal joint is disclosed in Japanese Patent Publication No. 59-17289 (1984) now patented.
With the foregoing kind of prior art boot for universal joint, when the joint is flexed at a joint angle (.theta.) of more than 30.degree. as illustrated in FIG. 4, however, the boot is deformed and deflexed in such a manner that the crest (6a) of a first ridge portion (6) of the bellows portion (7) close to the large-diameter portion (3) comes into point contact with the outer face (9) of the largediameter portion (3) at the axially inner end thereof, and as a result, the surface pressure on that area (9) locally increases. The large-diameter portion (3) and the first ridge portion (6) are equal in the angular velocity about the axis, but differ greatly in the peripheral velocity at the contact point owing to the differeince in radius. Consequently, cracking is liable to occur because of friction between both portions (3,6), with the result that longevity of the boot becomes shortened.
Further problem is that upon rotation of more than 1600 r.p.m., the grease hermetically filled in the space within the boot exerts a centrifugal action on the largest diameter zone of the bellows portion (7), namely the zone of it adjacent to the large-diameter portion (3) and the zone is dilated radially outwardly. As a result, the zone interferes in other component parts surrounding it nearby.
Therefore, any improvement in these problems over known boots for universal joint has been earnestly desired.